The primary function of a gear lubricant is to provide a high degree of reliability and durability in the service life of gear equipment. Gear lubricants may also contribute to improving the fuel economy of vehicles by improving the axle efficiency. See, for example, O'Connor et al., The Relationship Between Laboratory Axle Efficiency and Vehicle Fuel Consumption (SAE Paper No. 811206).
In the paper by O'Connor et al., entitled Axle Efficiency--Response to Synthetic Lubricant Components (SAE Paper No.821181), the authors state that "[i]nvestigations with both partial- and full-synthetic base formulations have shown improvements compared to conventional petroleum base gear oils. Maximum benefits are gained with total synthetic base type formulations."
Limited slip differentials are designed to restrict differentiation in a vehicle operating on a slippery surface. The limited slip characteristic is obtained by modifying a standard differential with the addition of a clutch. This clutch has the property of forcing both axle shafts to turn with the ring gear when the vehicle operates on a slippery surface. Limited slip differentials contain a slow-moving clutch. At low sliding velocities this clutch is prone to stick and then slip in a repetitive fashion unless a lubricant with the proper frictional characteristics is used. This stick-slip effect is very objectionable as it can result in loud chatter noises and severe vibration. The paper by John W. Allen, entitled Lubricants for Limited Slip Differentials (SAE Paper No. 660779), provides some background on the problems associated with limited slip differentials and some proposed lubricant solutions. The Allen paper does not teach or suggest the additives of the present invention or their use in lubricant formulations.
Power dividers are the linkages in the drivetrain that direct engine torque to gripping wheels rather than slipping wheels. The power divider's application is similar to the limited slip clutches in light duty axles. There are many types of power dividers, their overall purpose is to transmit torque to both sets of wheels or between the front and rear axles. In one particular design, this is accomplished by using a set of wedges between two cylindrical cams whose mating surfaces with the wedges are lobed. These lock for transmittal but slide to avoid torque buildup. When too much torque has built up without sliding, the wedges break the momentary welds formed. This is accompanied by a loud snap that can propel the truck sideways. Malfunctioning power dividers can result in broken axles.
Hutchison et al., in U.S. Pat. No. 4,948,523, discloses a lubricating composition that contains a silver protective agent. The silver protective agent comprises the reaction product of a C.sub.5 -C.sub.60 carboxylic acid and at least one amine selected from the group consisting of: 1) guanidine, urea and thioruea compounds; 2) C.sub.1 -C.sub.20 hydrocarbyl or hydroxy-substituted hydrocarbyl mono-amines, alkylene diamines; and 3) polyalkylene polyamines and N-alkyl glycine. This patent is directed to lubricating oil additives for medium speed diesel engines, such as locomotive engines, which have silver parts in the engine. Large, medium-speed diesel engines often contain silver protected components, such as bearings, and, as such, the lubricating oils may not contain the typical zinc containing wear inhibitors which attack the silver coated parts. This patent does not teach the use of the reaction products of the present invention in gear oil formulations or the improvements in, for example, axle efficiency, limited slip performance or power divider performance exhibited by the compositions of the present invention.